10:45 AM - 12:15 PM
[SCG58-P09] Silica deficiency uncouples the dissolution-precipitation process in plagioclase replacement
Keywords:plagioclase alteration, silica, psuedomorphic replacement, fluid chemistry
Feldspar replacement is commonly found in the hydrothermally altered crust including around the subduction zone. That replacement is characterized psuedomorphism and significant microporosity generation which inferred as an important fluid pathway within the crust. However, the effects of fluid compositions on the extent of psuedomorphism and microporosity formation in solid-solution plagioclase are still poorly understood.
In this study, the hydrothermal experiments on plagioclase replacement were conducted using pairs of starting minerals (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99; hornblende schist with labradorite An67-78Ab33-28) and fluid compositions (2M KCl and/or NaCl aqueous solutions, with or without quartz) with W/R=2. The vessel condition is 600°C and 150 MPa for 2-8 days experiment.
The plagioclase replacement proceeds faster in KCl, aq (up to 15.55 % reacted area per day) than NaCl, aq (1.2 % reacted area per day). The degree of albite replacement did not affect by presence of quartz, whereas anorthite is almost unaltered without quartz. In all runs, porosity with the size of 0.1 to 5 µm was produced mostly at the reaction front. Interestingly, Labradorite-KCl, aq without quartz generate the most intense pores formations (up to 7.7% porosity per day) hosted by secondary anorthite coupled with euhedral overgrowth of K-feldspar. The thermodynamic calculation on the fluid-mineral equilibria suggest that such replacement is explained by the high mobility of Na-Al and the immobility of Ca. However, the shift in the Raman active vibrations of 18O water enriched anorthite and K-feldspar reveal the oxygen silicate framework of labradorite was rearranged to form anorthite. Silica deficiency in solutions enhances the preferential dissolution of albite components from labradorite and precipitation of anorthite, and the remaining silica and aluminum components locally transported and encountered with K+ ions at the outsides of the crystals.
Our results represent an example that the silica deficiency in the fluids caused the differences in relative ion activity products of albite, anorthite and K-feldspars. This indicate that fluid chemistry may uncouple dissolution-precipitation mechanism, obstruct pseudomorhism, and intensify the porosity generation in crustal rocks.
In this study, the hydrothermal experiments on plagioclase replacement were conducted using pairs of starting minerals (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99; hornblende schist with labradorite An67-78Ab33-28) and fluid compositions (2M KCl and/or NaCl aqueous solutions, with or without quartz) with W/R=2. The vessel condition is 600°C and 150 MPa for 2-8 days experiment.
The plagioclase replacement proceeds faster in KCl, aq (up to 15.55 % reacted area per day) than NaCl, aq (1.2 % reacted area per day). The degree of albite replacement did not affect by presence of quartz, whereas anorthite is almost unaltered without quartz. In all runs, porosity with the size of 0.1 to 5 µm was produced mostly at the reaction front. Interestingly, Labradorite-KCl, aq without quartz generate the most intense pores formations (up to 7.7% porosity per day) hosted by secondary anorthite coupled with euhedral overgrowth of K-feldspar. The thermodynamic calculation on the fluid-mineral equilibria suggest that such replacement is explained by the high mobility of Na-Al and the immobility of Ca. However, the shift in the Raman active vibrations of 18O water enriched anorthite and K-feldspar reveal the oxygen silicate framework of labradorite was rearranged to form anorthite. Silica deficiency in solutions enhances the preferential dissolution of albite components from labradorite and precipitation of anorthite, and the remaining silica and aluminum components locally transported and encountered with K+ ions at the outsides of the crystals.
Our results represent an example that the silica deficiency in the fluids caused the differences in relative ion activity products of albite, anorthite and K-feldspars. This indicate that fluid chemistry may uncouple dissolution-precipitation mechanism, obstruct pseudomorhism, and intensify the porosity generation in crustal rocks.